Relapsing fever spirochetosis is a global disease transmitted by argasid (soft-shelled) ticks or the human body louse. Throughout the world, this disease causes severe morbidity and mortality if left untreated, primarily affecting those in resource poor countries. However, little is known regarding the molecular mechanisms involved with transmission of the spirochetes from the tick vector to the vertebrate host. Therefore, to address NIAID's role in improving global health, my goal for this proposal is to identify the genetic factors that enable a species of relapsing fever spirochetes, Borrelia turicatae, to colonize the tick and to be subsequently transmitted to the mammalian host. Furthermore, there are several distinctions between B. turicatae and Lyme disease-causing spirochetes which make B. turicatae advantageous for studying spirochete-host interactions. These differences include plasmid stability and retention of infectivity after prolonged in vitro cultivation, and the ability to reach high bacterial densities in mammalian blood. These distinctions have aided in the development of the proposed studies focusing on determining the necessity of outer membrane proteins (Omps) during tick colonization and transmission of B. turicatae from its tick vector, Ornithodoros turicata. The importance of a family of genes which code for Omps has been established in a different species of relapsing fever spirochetes, Borrelia hermsii. However, little is known about the role of Omps in other species that cause disease. Therefore, I propose to identify genes which code for Omps that are up-regulated within B. turicatae during tick infection. A microarray based approach will be used to identify these genetic determinants, and their necessity for tick colonization and transmission to the mammal will be determined. To accomplish this, I have developed the first system in B. turicatae for gene inactivation and complementation, and have established the only known laboratory tick colony for these studies. These findings will have a broad impact enabling the development of therapeutics blocking transmission of this disease, and will increase our understanding of the adaptation and transmission mechanisms of vector-borne pathogens.